Many scientific instruments and their components, such as optical surfaces to detectors used in space research and exploration, are prone to contamination from organic molecules. Over a period of time, contaminant build up can adversely affect the performance of such instruments. Of particular concern are cryogenically cooled image processing optics and sensors because contaminant adsorption in these devices causes a reduction in the quality of the information transmitted from space.
Small organic contaminants, (molecules with a molecular size of about 6 .ANG. or less) such as straight chain hydrocarbons, benzene, and their simple derivatives, are particularly difficult to remove from gas samples. Such hydrocarbon contaminants may come from paints, plastics, adhesives, motor or magnet windings, and like sources. Other sources may include combustion and biodegradation products, wastewater and environmental remediation processes.
Previously, control of organic contaminants has involved the use of adsorbents such as aluminosilicates which have a pore size on the order of 10 .ANG., such as Linde zeolite 13X and LZY52, and hydrophobic silica such as SILICALITE.TM., a commercially available molecular sieving material with an average pore size greater than 5 .ANG.. However, 13X and LZY52 aluminosilicates are not effective adsorbents for smaller organics. Smaller pore size aluminosilicates, such as zeolite 4A are not hydrophobic. Further, aluminosilicates tend to adsorb large quantities of water vapor, lessening the capacity of the adsorbent for trapping the organics.
Hydrophobic silica, such as SILICALITE.TM. which has a pore size greater than 5 .ANG., although highly hydrophobic, does not effectively adsorb small organic molecules (i.e., molecules with a molecular size between 3 and 5 .ANG.). Rather, SILICALITE.TM. preferentially adsorbs molecules with a molecular size of about 5-6 .ANG.. This can be explained by the observation that adsorbents tend to adsorb molecules with a molecular size near that of the adsorbent pore size most strongly. Thus, the composite adsorbent possesses the adsorptive size selectivity from 3-6 .ANG..